Systems of the type under consideration normally comprise miniature electronic apparatus for determining the relative direction of a remote source of radio transmissions or comprise perimeter alarm apparatus including a loop of wire to be placed around an area to be guarded and electronic circuitry to be connected with the loop of wire for detecting a signal imposed on the loop from a remote transmitter.
One example of miniaturized electronic apparatus for determining the relative direction to a remote source of radio transmissions is U.S. Pat. No. 3,336,530.
This patent discloses a radio transmitter/receiver tracking system using modulated radio transmissions to provide information of the activity at the location of a remote radio transmitting source, and includes a directional antenna to determine the general direction from which the radio transmissions are being received.
U.S. Pat. No. 4,136,338, discloses a transmitter/monitor system which signals an alarm when the transmitter is moved outside a given boundary. The boundary comprises a loop of wire placed around an area to be guarded and electronic circuitry connected to the wire loop for detecting signals imposed on the loop. A transmitter within the bounded area provides the signal imposed on the loop. Once the transmitter no longer imposes the signal on the loop, the alarm circuitry is energized and an alarm sounds.
In this prior art alternative the transmitter includes a sequencer in connection with three oscillators which alternately energizes coils to create magnetic flux lines.
The monitor includes a perimeter loop connected with a bandpass filter and amplifier circuit which is connected to a frequency and level detector which operates a time delay circuit to energize an alarm when the necessary signal is absent for a predetermined period of time.
The perimeter loop and the transmitter are coupled by magnetic induction, upon energization of the coils.